Moisture is an important issue in many air systems, including ambient air. Its content in controlled air systems must be carefully measured and controlled through use of dew point sensors. The use of dew point temperature of a volume of air (expressed in ° F. or ° C.) to indicate the amount of moisture in it is based upon the operational principle of cooling a volume of air and monitoring the temperature of a surface when condensation first forms on it. The three most common types of sensors for measuring dew point today are chilled mirrors, metal oxide and polymer sensors.
Of the three most common types of dew point sensors, chilled mirror technology can offer the highest accuracy over a wide range of dew points. The operational principle is based on the fundamental definition of dew point—cooling a volume of air until condensation forms. A gas sample passes over a metallic mirror surface which is chilled by a cooler. Light is then directed at the mirror allowing an optical sensor to measure the amount of reflected light. When the mirror is cooled to the point at which condensation begins to form on its surface (i.e. the dew point has been reached), the amount of light reflected by the mirror diminishes which is in turn detected by an optical sensor. The rate of cooling is then carefully regulated by a temperature sensor on the mirror. Once a state of equilibrium has been reached between the rate of evaporation and condensation, the mirror temperature is equal to the dew point. Although chilled mirror technology offers the highest accuracy in dew point measurements, a dew point sensor designed around it is in essence a very delicate and precision optical instrument. Its optical measurement principle is highly sensitive to the presence of dirt, oil, dust and other contaminants on the mirror surface. In addition, to correctly calibrate such a precision optical instrument requires not only expertly trained know-how in this technology field with years of experience but also a lot of time and effort required to be spent in doing it. It is therefore not surprising that chilled mirror dew point sensors are extremely expensive and are only employed when absolute accuracy is essential. Furthermore, frequent maintenance, cleaning and calibration must also be regularly performed on them in order for them to function properly.
Capacitive metal oxide sensors, including aluminum oxide technology, are designed for very low dew point measurement in industrial processes. While the types of materials used in construction can vary, the sensor structure and operating principle generally remain the same. Capacitive sensors are built in a layered structure sandwiching together a substrate base layer, a lower electrode, a hygroscopic metal-oxide middle layer, and a water permeable upper electrode. The capacitance across the upper and lower electrode changes based on the amount of water vapor absorbed by the metal oxide layer (the dielectric of the capacitor), which is a function of dew point. While capacitive sensors provide excellent low dew point measurement accuracy to −100° C. and lower, they tend to offer poor long-term stability in processes with varying dew points at the higher ranges (e.g. refrigerant dried systems). Metal oxide sensors can also be easily damaged by high humidity levels and condensation. Drift in the output reading means frequent calibration which requires rather sophisticated and complex instrumentation that can only be done at the manufacturer's calibration laboratories.
Capacitive polymer sensors measure accurately over a wider humidity range than metal oxide counterparts and, although still not totally satisfactory, also offer better long-term stability than capacitive metal oxide sensors. While the capacitive operating principle is similar to that of metal oxide, there are a few key differences. Beyond the obvious material difference in the hygroscopic layer (polymer vs. metal oxide), a capacitive polymer sensor is also bonded together with a resistive temperature sensor. The polymer sensor measures the humidity (amount of water molecules in the measured gas) in terms of relative humidity (RH) while the temperature sensor measures the temperature of the polymer sensor. From these two values, the microprocessor in the transmitter electronics calculates the dew point temperature. Like the capacitive metal oxide sensors, calibration of capacitive polymer sensors also requires sophisticated and complex instrumentation and can only be performed at the manufacturer's laboratories prior to shipping to distributors or end-users.
It can be seen from the foregoing discussion that just about all dew point sensors available for sale today require rather elaborate and time-consuming calibration procedures at the factory prior to shipment to customers. With the exception of chilled mirror sensors, the metal oxide and polymer sensors typically have a rod-like shaped cylindrical probe as part of the sensor. In order to overcome the inconvenience of having to ship these sensors back to the factory for re-calibration if needed, portable high-precision humidity calibrators are available for calibrating metal oxide and polymer sensor probes in the field. The most common one widely used in the field is the two-pressure reactor which is similar in design to precision humidity calibration instruments used in national bureaus for standards. Air or nitrogen at a pressure P1 is led through a chamber partially filled with water and saturated to 100% relative humidity (RH) at P1. By means of a reduction valve, the saturated air is reduced to ambient pressure Pa and fed into a measurement chamber. By design the saturation chamber and the measurement chamber are accurately maintained at the same temperature of Ta. Under these conditions, the water-vapor partial pressure Pw is reduced from the saturated vapor pressure Psw at the same ratio as the total pressure orPw=Psw×(Pa/P1)From this it follows that:RH=Pw/Psw=Pa/P1 at temperature Ta. Thus, the generated relative humidity essentially depends on the ratio of the two pressures.
Despite the fact that a two-pressure reactor can be used to adequately calibrate metal oxide and polymer dew point sensors away from their factories, this instrument is very expensive, typically running between US$20,000.00 to US$30,000.00. Furthermore, experienced technicians are required to operate such an instrument and the time it takes to calibrate a metal oxide or polymer dew point sensor easily runs into two or more hours per probe.
Non-dispersive infrared (NDIR) gas sensors have been considered as one of the best methods for gas measurement since the 1950s. This method takes advantage of the fact that all gases vibrate at a unique frequency based upon their individual molecular makeup. The unique vibration of each type of gas molecule will absorb radiation at very specific and unique wavelengths in the infrared portion of the electromagnetic spectrum. The three most important gases in our atmosphere are Oxygen (O2), Water Vapor (H2O) and Carbon Dioxide (CO2). Since Oxygen has a symmetrical molecular structure, it has no infrared absorption bands available for use with the NDIR gas sensing methodology. Hence NDIR Oxygen sensors simply do not exist. On the other hand, NDIR CO2 sensors today can readily be found almost everywhere. But the most surprising fact is that NDIR H2O sensors or NDIR dew point sensors, irrespective of their cost, can hardly be found anywhere at all. Obviously their absence is not because water vapor is not an important gas in our atmosphere. Nor it is because of its improper molecular makeup. As a matter of fact, like CO2, H2O has several very strong and specific infrared absorption bands ideally suitable for use with NDIR gas sensing methodology, the most notable of which is at 2.73μ. So the logical question to ask at this point is why?
As it turns out, water vapor or H2O is by its nature a very unique and peculiar gas all because of the fact that it has a liquid phase, namely water, between the temperatures of zero (0° C.) and 100° C. No other gas in the atmosphere that we know today has liquid phases in this temperature range. Because of this unique occurrence, the presence or absence of H2O in an air volume strictly depends upon the latter's temperature relative to its surroundings. If the temperature of the air volume wherein H2O molecules find themselves in is higher than a physical surface nearby, H2O molecules will disappear from the air volume by condensing themselves onto the colder surface. If H2O molecules physically bound to a physical surface whose temperature is higher than that of the air volume surrounding it, the H2O molecules will disappear from the surface by evaporating themselves into the surrounding air volume. By the same token, how much H2O one finds above a water surface strictly depends upon the latter's temperature. Because of this unique behavior, H2O has a nickname and it is called a “Houdini gas”. This Houdini act of H2O would not have been so prominent if not for the fact that such an act can take place in the smallest of volume or surface areas approaching those of molecular sizes. Thus H2O can disappear to or appear from unimaginably small cracks, crevices or openings. It is in this so-called Houdini behavior of H2O that we are finally able to find the answer to the earlier question as to why NDIR dew point sensors can rarely or hardly be found anywhere at all.
Earlier it was mentioned that H2O has a very strong and specific absorption band at 2.73μ in the infrared spectrum. Similar to the case for CO2 gas, this absorption band of H2O can readily be used to design a simple, reliable and low cost NDIR H2O or dew point sensor. However, because of the fact that H2O is a Houdini gas and, unlike CO2, devising an accurate and dependable calibration procedure for NDIR H2O or dew point sensors is a very serious and expensive proposition. When calibrating an NDIR gas sensor, it is paramount that several gas standards spanning a desirable gas concentration range for the sensor are readily available. For example, if one wishes to calibrate an NDIR CO2 sensor having a measurement range from 0 to 5,000 ppm, one would need at least 6 or 7 gas standards in order to get the job done. These gas standards would probably have concentrations ranging from 0 to 5,000 ppm in increment steps of 1,000 ppm and they should be readily available. This is indeed the case for calibrating an NDIR CO2 sensor. However, for an NDIR H2O or dew point sensor, because H2O is a Houdini gas, preparing H2O gas standards needed for an accurate calibration procedure is not a simple task at all. This does not mean that adequate H2O gas standards cannot be properly prepared for the task at hand, it is only that the cost and efforts involved in such an undertaking are so prohibitively high as to render these sensors economically vastly disadvantageous when compared with metal oxide or polymer dew point sensors. This is precisely the reason why NDIR H2O or dew point sensors can rarely be found anywhere.
It is amply clear from the foregoing discussion that low cost NDIR dew point sensors if available for sale today to the public would fill a big vacuum left open by existing available dew point sensors in terms of sensor cost, performance, reliability, long term stability and life. It is the object of the current invention to advance a novel methodology for calibrating NDIR dew point sensors without the need to use the difficult-to-prepare H2O standards. By so doing, the prohibitively high calibration cost for NDIR dew point sensors will be eliminated thereby opening the door for the possibility of producing such low cost NDIR dew point sensors in the future.